The pg_freespacemap module provides
a means for examining the free space map (FSM). It provides a
function called pg_freespace, or
two overloaded functions, to be precise. The functions show the
value recorded in the free space map for a given page, or for all
pages in the relation.

By default public access is revoked from the functions, just
in case there are security issues lurking.

Returns the amount of free space on the page of the
relation, specified by blkno,
according to the FSM.

pg_freespace(rel regclass IN,
blkno OUT bigint, avail OUT int2)

Displays the amount of free space on each page of the
relation, according to the FSM. A set of (blkno bigint, avail int2) tuples is
returned, one tuple for each page in the relation.

The values stored in the free space map are not exact.
They're rounded to precision of 1/256th of BLCKSZ (32 bytes with default BLCKSZ), and they're not kept fully up-to-date as
tuples are inserted and updated.

For indexes, what is tracked is entirely-unused pages,
rather than free space within pages. Therefore, the values are
not meaningful, just whether a page is full or empty.

Note: The interface was changed in version 8.4,
to reflect the new FSM implementation introduced in the
same version.